This invention relates to a method of manufacturing an impregnated cathode and to an impregnated cathode manufactured by the method.
An impregnated cathode is preferred in a thermoelectronic tube, such as a highly reliable microwave tube for use in satellite communication, a linear accelerator, or a highly resolving image pickup or display tube which is under progress for new media. The impregnated cathode includes an electron emissive or emission active substance in a porous matrix of a refractory metal and has a high emission current density and a long life. It is believed that this is because a monoatomic layer of free barium is formed as a thermoelectron emissive surface of the cathode and is quickly replenished by diffusion of the electron emissive substance from the matrix.
The impregnated cathode may be an impregnated dispenser cathode disclosed in U.S. Pat. No. 3,358,178 issued to Avraam I. Figner and two others or in U.S. Pat. No. 4,165,473 issued to Louis R. Falce and assigned to Varian Associates, Inc., California. In the manner which will later be described a little more in detail, the conventional method of manufacturing such as impregnated cathode is defective.
An improved method of manufacturing an impregnated cathode is therefore revealed in U.S. Pat. No. 5,096,450 issued to Toshikazu Sugimura, the present inventor, and four others. According to the improved method, powder of an electron emissive substance is first prepared by mixing powder of barium carbonate, calcium carbonate, and aluminium oxide into fixed powder, firing the mixed powder into fired powder, and crushing the fired powder into the powder of the electron emissive substance. Metal powder of a high melting point and a heat resistive property and the powder of the electron emissive substance are now mixed in a dry state into cathode forming powder. The cathode forming powder is press-shaped into a shaped body. The shaped body is sealed in a glass reaction vessel and is subjected to a hot isostatic pressing (HIP) treatment with the sealed vessel placed in an argon atmosphere of a substantially constant final temperature between 1000.degree. C. and 1300.degree. C. and of 1500 atmospheres (atm) for ninety minutes. The shaped body is thereby changed to a sintered body of the cathode forming powder.
It is possible with the improved method to remove the defects of the conventional method. The instant inventor has, however, found that barium oxide is liable to react with tungsten used as the metal during the hot isostatic press treatment to become barium tungstate (BaWO.sub.4) if used as the electron emissive substance. This adversely affects formation of the monoatomic layer of free barium. Furthermore, the inventor has found that carbon in a carbonate reacts with tungsten during the hot isostatic press treatment to become tungsten carbide (WC). This reaction takes place if barium carbonate were included in the electron emissive substance although the electron emissive substance includes theoretically no barium carbonate. If formed, the tungsten carbide adversely affects the reduction reaction which is indispensable for thermoelectron emission and is otherwise duly caused by the tungsten included in the sintered body as a matrix.